1. Field of the Invention
The invention relates to a method for operating a Coriolis mass flowmeter with at least one actuator assembly, wherein the actuator assembly includes a coil with a first conductor and a second conductor at least windings of the first conductor and windings of the second conductor running parallel to each other, and wherein the first conductor and the second conductor are insulated from each other at least in the area of the windings.
2. Description of Related Art
Mass flowmeters using the Coriolis principle have been known for many years and allow for determination of the mass flow rate of the medium flowing through the measuring tube with high accuracy. In order to determine the mass flow rate, the measuring tube having an actuator assembly or also multiple actuator assemblies is excited to oscillation—in particular, with the eigenfrequency of a certain eigenform of oscillation—and the resulting oscillations are detected by sensor assemblies and then evaluated. The oscillation excitation of the measuring tube and the detection of the oscillation can occur in different manners. According to a common method, the actuator assembly and often also the sensor assembly are designed in such a manner that they have an adjusting element as well as a coil, wherein the adjusting element is moved in the case of the actuator assembly by a magnetic field of the corresponding activated coil, which leads to the measuring tube being deflected electromagnetically and brought into oscillation. In the case of the sensor assembly, the adjusting element provided with a permanent magnet is deflected by the movement of the measuring tube and an electrical voltage is induced by the movement of the magnet in the coil formed by the windings of the conductor. By evaluating this electrical voltage, the movement of the adjusting element and thus the oscillation of the measuring tube can be detected.
The evaluation of the measuring results occurs in that a phase shift is determined between oscillations detected by two offset sensor assemblies arranged on the measuring tube, wherein this phase shift is a dimension for the mass flow rate. Coriolis mass flowmeter are known that have one, single measuring tube, but also that have a plurality of measuring tubes or exactly two measuring tubes, wherein the measuring tubes are either essentially straight or curved; these constructive differences are not of importance for the present invention.
Coriolis mass flowmeters are resonance-measuring systems, whose measuring tubes are constantly moving as necessary for operation. The elements directly or indirectly in contact with the measuring tubes are thus exposed to continuous vibration, this is true, in particular, for the actuator assemblies as well as for the sensor assemblies. The windings of the first conductor of the actuator assembly finally forming an electrical coil and therefore represent an electrical energy storage. In a cable break in or on the coil caused by constant vibration, it is possible for a break to occur in the conductor, wherein this leads to a flashover due to the constant flow of current in the energy storage. When using Coriolis mass flowmeters in explosive surroundings, suitable measures need to be taken to avoid a flashover in the area of the coil of the actuator assembly of the Coriolis mass flowmeter. In addition to reducing the intensity of the current, which, however, goes hand in hand with a reduction of the intensity of the magnetic field and thus the power for oscillation excitation, such a measure can exist, for example, in casting of the coil, which is complex and increases the costs for production. In particular, in Coriolis mass flowmeters having measuring tubes with a large nominal width or in using Coriolis mass flowmeters with media having strongly damping characteristics, large coils with many windings are used, which have high energy during operation, thus increasing the risk of a flashover.